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Hindawi Publishing CorporationJournal of NanomaterialsVolume 2012, Article ID 237853, 14 pagesdoi:10.1155/2012/237853

Research Article

Influence of Synthesis Conditions on the PhysicochemicalProperties and Catalytic Activity of Fe/Cr-Pillared Bentonites

Fatma Tomul

Department of Science Education, Education Faculty, Mehmet Akif Ersoy University, 15100 Burdur, Turkey

Correspondence should be addressed to Fatma Tomul, [email protected]

Received 13 September 2011; Accepted 7 October 2011

Academic Editor: Anukorn Phuruangrat

Copyright © 2012 Fatma Tomul. This is an open access article distributed under the Creative Commons Attribution License, whichpermits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

The synthesis of Fe/Cr-pillared bentonites starting from a 2% clay suspension and also from dry clay using ultrasound treatmentfor both the aging and the intercalation steps of the pillaring solution considerably reduces the time and the amount of waterrequired compared with the conventional synthesis method. The catalysts were characterized using scanning electron microscopywith an energy dispersive system (SEM-EDS), powder X-ray diffraction (XRD), N2 and CO2-adsorption/desorption, electronparamagnetic resonance (EPR), Fourier-transform infrared spectroscopy (FTIR) and thermogravimetric (TG) analysis and thecatalytic activity of selected samples was evaluated for the phenol oxidation reaction. The results of XRD analysis showed thatdelaminated Fe/Cr-pillared bentonite with d001 value 68 A was observed after intercalation a direct mixture of the pillaring solutionand dry clay. The adsorption-desorption isotherm analysis showed that the samples synthesized with the proposed methodologyin intercalation stage have similar textural properties and these properties do not change remarkably with synthesis conditions.In addition, the characterisation studies showed that the physicochemical properties of samples synthesized by ultrasound werecomparable to those of sample synthesized by conventional method in this study. The sample synthesized by conventional methodshow 50% phenol conversion. This value was higher than those of samples synthesized by the ultrasound.

1. Introduction

Pillared clays have been studied as potential micro/meso-porous materials for use as catalysts and catalyst supportson the industrial scale because of their well-controlledpore structure and catalytic properties [1–4]. However,the excessive amount of water required for the dilutedclay suspensions and pillaring agents, combined with longsynthesis times for the pillared clays, makes it difficult toproduce them on the industrial scale [5, 6]. For this reason,studies related to developing a more rapid synthesis of thesematerials have recently become important. At this point inthe development of pillared clay catalysts for the industrialscale, it is important to improve their physicochemical andcatalytic properties as well as to develop synthesis methodsthat decrease their production time and water use [6–10].

The literature indicates that catalysts that include chro-mium and/or iron give good results for reactions such asmethanol dehydration [11], ethylbenzene dehydrogenation

[12], Fischer Tropsch synthesis [13], aromatic nitration, thetoluene disproportionation [14], carbon monoxide oxida-tion [15, 16], catalytic acylation of alcohols [17], and phenoloxidation [6, 18–21].

In the literature, the studies related to iron/chromium-mixed pillared clay are especially limited in number, eventhough it is well known that these metals provide superiorresults for various catalytic reactions. Chromium is prefer-able for catalytic applications because of its large range ofoxidation numbers despite the major disadvantage that thepillared structure of the chromium-pillared clays is damagedwhen calcined at high temperatures, decreasing the surfacearea [22, 23]. The disadvantage of Fe-pillared interlayeredclays is the result of their small basal spacing, which couldadversely impact their surface areas and pore properties andthus reduce their reaction activities [24]. Related studies haveimplied that catalytic activity can be increased by adding asecond metal to the structure of the pillared clays producedusing the iron pillaring solution [25].

2 Journal of Nanomaterials

Table 1: Preparative conditions and nomenclature of the solids obtained.

Nomenclature Fe/Cr ratio Fe/Fe + Cr ratio Aging of pillaring solution Intercalation

Fe/Cr0.75 15 : 5 0.75Fe/Cr pillaring solution aged for 24 h atroom temperature

Fe/Cr pillaring solution slowly added to2% bentonite suspension and aged for24 h at room temperature

Fe/Cr0.75(D) 15 : 5 0.75Fe/Cr pillaring solution sonicated for 30minutes at 35 Hz and 75◦C

Dry clay directly added to Fe/Cr-pillaringsolution sonicated for 30 minutes at 35 Hzand 75◦C

Fe/Cr0.75(CS) 15 : 5 0.75Fe/Cr pillaring solution sonicated for 30minutes at 35 Hz and 75◦C

Dry clay slowly added to Fe/Cr-pillaringsolution sonicated for 30 minutes at 35 Hzand 75◦C

Fe/Cr0.75(SS) 15 : 5 0.75Fe/Cr pillaring solution sonicated for 30minutes at 35 Hz and 75◦C

Fe/Cr pillaring solution slowly added to2% bentonite suspension sonicated for 30minutes at 35 Hz and 75◦C

Fe/Cr0.25(D) 5 : 15 0.25Fe/Cr pillaring solution sonicated for 30minutes at 35 Hz and 75◦C

Dry clay directly added to Fe/Cr-pillaringsolution sonicated for 30 minutes at 35 Hzand 75◦C

Phenol is an important and representative temperatureresistant organic pollutant that is toxic even at low concen-trations and, if found in natural water, generates harmfulchlorine compounds during the disinfection of natural watervia chlorination [6, 20, 26]. For this reason, it has becomeincreasingly important to purify water contaminated byphenol and its derivatives in recent years. It seemed to bemore effective to use advanced oxidation processes to purifywaste water containing organic pollutants that are difficult orimpossible to decompose biologically [27]. Among advancedoxidation processes, the activation of hydrogen peroxide bymeans of a solid catalyst (catalytic wet peroxide oxidationCWPO) is the most promising process, both economicallyand technologically. Pillared clays have offered an importantline of research in obtaining solids responding to the CWPOprocess [15].

In this study, synthesis of iron/chromium-pillared ben-tonites was accomplished by using ultrasound treatmentduring both the aging and the intercalation of the pillaringsolution. Furthermore, during intercalation stage the pillar-ing solution and clay are mixed in three different ways: (1) byadding the pillaring solution directly to the dry clay, (2) byadding the dry clay slowly to the pillaring solution, and (3) byadding the pillaring solution slowly to a 2% clay suspension.The physicochemical properties of the derived samples wereanalyzed by SEM-EDS, XRD, nitrogen and carbon dioxideadsorption/desorption, EPR, FTIR, and TG analysis. Thecatalytic activities of the Fe/Cr-pillared bentonites wereassessed for the catalytic wet peroxide oxidation of phenol.

2. Materials and Methods

2.1. Materials. Raw bentonite clay (RB) was obtained fromUnye (Turkey) and was used without any further purificationor cation exchange. All the chemicals, from Sigma-Aldrichand Merck, were of laboratory reagent grade and usedwithout further purification. Doubly distilled water was usedthroughout the work.

2.2. Synthesis of Fe/Cr-Pillared Bentonites. Fe/Cr-pillaredbentonite synthesis was carried out, with any indicatedmodifications, according to the procedure of Olaya et al.[9]. Pillaring solutions containing iron and chromium wereprepared via dropwise addition (flow rate of 1 mL/minute)of a 0.4 M NaOH solution into a solution of FeCl3·6H2Oand CrCl3·6H2O salts in either a 15 : 5 or 5 : 15 Fe : Cr molarratio, producing on OH−/Fe3+ + Cr3+ ratio of 2.4. Fe/Cr-pillaring solutions were aged with ultrasound at 75◦C for30 minutes, used an ultrasonic bath (Bandelin Sanorex,operating frequency 35 kHz). The intercalation process wasperformed using one of three methods: (1) by directly mixingthe pillaring solution and the dry clay, (2) by slowly addingthe dry clay to the pillaring solution, or (3) by slowlyadding the pillaring solution to a 2% suspension at of theclay to make a 20 mmol ratio of Fe + Cr/g of clay. Thederived suspensions were separated from the liquid phaseby centrifugation, and then they were washed to removethe chlorine ions and calcined at 300◦C (heating ramp of3◦C/min) for 3 hours after drying at 60◦C for 16 hours.For a comparison, Fe/Cr-pillared bentonite was prepared byconventional method. Fe/Cr-pillaring solution was preparedto make Fe/Cr ratio of 15 : 5 and an OH/Fe + Cr ratioof 2.4 and kept at room temperature for 24 hours beforeadding it dropwise to the previously prepared 2% m/msuspension to give a 20 mmol ratio of Fe + Cr/g bentonite,which was then kept at room temperature for 24 hours withstirring. Filtration, drying, and calcination were performedas previously described. The conditions of synthesis and thenomenclature of the solids obtained were summarized inTable 1.

2.3. Characterization Studies. Scanning electron microscopy(SEM) microphotographs were obtained from powderedsamples with a QUANTA 400F Field Emission SEM. Thechemical composition of some of the pillared bentoniteswas determined using a QUANTA 400F Field Emission SEMEnergy Dispersive X-ray Spectrometer (EDS).

Journal of Nanomaterials 3

Table 2: Chemical properties of raw and pillared bentonites.

Sample codeMetal oxides % m/m

SiO2 Al2O3 MgO CaO K2O Fe2O3 Cr2O3

Raw bentonite 68.1 24.3 4.52 0.79 0.33 2.04 0.00

Fe/Cr0.75 63.3 22.5 4.00 0.00 0.27 5.23 4.71

Fe/Cr0.75(D) 65.2 21.9 4.41 0.00 0.29 3.14 5.01

Fe/Cr0.75(SS) 61.8 21.2 4.41 0.00 0.23 7.48 4.81

Fe/Cr0.25(D) 64.4 20.0 4.47 0.00 0.23 3.52 7.18

The obtained solid products were characterized using X-ray diffraction on a Phillips PW 3710 diffractometer usingCu Kα radiation (40 kV, 40 mA) that ranged between 1and 70◦ 2θ. Semiquantitative percentages of the clay werecalculated by means of mineral intensity factors, as suggestedby Yalcin and Bozkaya [28], and based on the externalstandard method of Brindley [29].

The porous structure of the samples was characterizedfrom nitrogen and carbon dioxide adsorption-desorptionisotherms. Nitrogen adsorption/desorption isotherms of thesamples were obtained by means of a QuanthrocromeNovaWin gas adsorption system at liquid nitrogen temper-ature. Carbon dioxide adsorption-desorption isotherms ofthe samples were obtained by means of a QuantachromeAutosorp 1C gas adsorption system at liquid carbon dioxidetemperature. Prior to the measurement, samples were out-gassed at a temperature of 250◦C under high vacuum for16 h. Specific BET surface area (SBET) values were calculatedwith 0.05 < P/P0 < 0.30. The specific external surfacearea (Sext) and the micropore volume (Vμ,t) were obtainedusing the t-plot method. The total pore volume (Vt) wasestimated from the adsorption data at a P/P0 value of ∼0.99.The Barrett-Joyner-Halenda (BJH) method was applied tothe desorption data for P/P0 values above 0.20 to determinethe mesopore surface area (SBJH) and the mesopore volume(VBJH) for pores in the 10–500 A range. The micropore andmesopore volume distribution as a function of pore size wascalculated by Horvath-Kawazoe (HK) and Barrett-Joyner-Halenda (BJH) method, respectively. Micropore volume(Vμ,HK) values were calculated by Horvath-Kawazoe (HK)method, which was applied for micropore size distribution[30, 31].

The Electron paramagnetic resonance (EPR) spectrawere recorded in a continuous wave mode using the X bandat room temperature using a Bruker ELEXSYS E580 spectro-meter.

The FTIR spectra were obtained using KBr pellets anda Perkin Elmer BX-FTIR spectrometer, in the range of 400–4000 cm−1. All spectra were collected at room temperature,with a resolution of 4 cm−1. The acidity of the OH-groupswas determined by collecting the spectra of the samples aftersaturating them with benzene by exposing them to benzenesteam for 48 hours.

Thermogravimetric analysis (TGA) was performed byheating the samples in air with a flow rate of 80 mL/minusing a Setaram Labsys TGA/DTA thermal analyzer with aheating rate of 10◦C/min.

2.4. Catalytic Studies. The catalytic experiments of the wetperoxide oxidation of phenol were performed using a glassbatch reactor with a capacity of 500 mL that had been opento the atmosphere, thermostated at 298 K, and thoroughlystirred. The reactor was charged with 100 mL of 50 ppmphenol solution and a catalyst (typically, 5.0 g/L). Whenthe required temperature was achieved, the correspondingamount of hydrogen peroxide (0.1 M, 6 mL) was addedto the reactor to start the reaction. The total reactiontime was fixed at 3 hours. Samples were drawn at 60 and180 minutes and were filtered through a 0.22 μm nylonfilter. The concentration of phenol was determined usingthe colorimetric reaction of the samples with the Folin-Ciocalteau (F-C) reagent (Fluka) based on the test proceduredeveloped by Box [32]. TOC analyses were performed on aShimadzu VCPN model carbon analyzer (via the combustionmethod) that had been equipped with an autosampler.

At the end of each trial, a solution was separated fromthe reactive medium by filtration in order to determine theleaching of iron and chromium during the reaction. Theanalysis of iron and chromium in solution was determinedby atomic absorption, using a Perkin Elmer AA800 Modelspectrometer.

3. Results and Discussion

3.1. Analysis of SEM-EDS. The scanning electron micro-graphs of both raw and some pillared bentonites are shown inFigure 1. Some variation in the shape and surface structure ofthe bentonites, which occurred as a result of converting theraw bentonite to the pillared bentonite, can be seen in theSEM micrographs. The samples Fe/Cr0.75 and Fe/Cr0.25(D)show that there was both greater aggregation and greatercohesion than for the raw bentonites and that the poresbetween the particles were closed. However, it is seen that thenumber of pores between the particles increased for both thesamples Fe/Cr0.75(D) and Fe/Cr 0.75(SS). This observationindicated that the use of ultrasound during intercalationfavors the formation of larger pores.

The chemical composition of the raw and the pillaredbentonites is reported in Table 2. The pillaring of the rawbentonite by Fe/Cr pillars resulted in an increase in theFe2O3 and Cr2O3 content, with complete replacement of theinterlayer Ca cations. This increase in the Fe2O3 and Cr2O3

content, with a corresponding decrease in the number ofexchangeable Ca cations, indicates the successive replace-ment of interlayer cations with Fe/Cr pillars. Moreover, it is


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Figure 1: SEM microphotographs of (a) RB, (b) Fe/Cr0.75, (c) Fe/Cr0.75(D), (d) Fe/Cr0.75(SS), and (e) Fe/Cr0.25(D) samples.


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Figure 2: X-ray diffraction patterns of pillared bentonite samples. (a) Fe/Cr-intercalated-pillared bentonites. (b) Fe/Cr-pillared bentonites.

observed that the amount of iron and chromium incorpo-rated into the structure during intercalation depended onthe procedure being applied and treatment. Fe/Cr0.75(SS)includes more iron than other samples, with the sameFe/Fe + Cr ratio, prepared by conventional and ultra-sound treatments. This observation indicates that there isa beneficial effect of using ultrasound treatment and byadding the pillaring solution slowly to the clay suspensionon amount of iron that incorporated into the structure.The use of ultrasound in the intercalation stage probablyincreases exchange of pillars and improves the dispersion andhomogenization of the suspended solids [10]. Furthermore,from EDS analysis results, it was observed that, by changingthe Fe/Fe + Cr ratio in the pillaring solution, the Fe/Crratio in pillared clay could be changed; the success of thereplacement of iron and chromium in the structure wasincreased by pillaring with a solution containing an Fe/Fe +Cr ratio of 0.75.

3.2. X-Ray Diffraction Patterns. The XRD patterns of Fe/Cr-pillared bentonites, together with those of raw and Fe/Cr-intercalated bentonites, are shown in Figure 2. Basal spacing(d001) values are summarized in Table 3. The reflection angle(2θ) of raw bentonite at the 001 plane is determined as5.65◦, corresponding to a basal spacing (d001) value of 15.6 A.In accordance with the results found in the literature, it is

Table 3: Basal spacing (d001) values of raw and intercalated/pillaredbentonites.

Sample coded001 (A)

Intercalated Pillared

Raw bentonite 15.6 14.7

Fe/Cr0.75 15.6 —

Fe/Cr0.75(D) 68.0–15.7 54.2–13.7

Fe/Cr0.75(CS) 15.6 —

Fe/Cr0.75(SS) 15.8 14.1

Fe/Cr0.25(D) 17.3 12.0

observed that the bentonite samples possessed the bentonitecharacteristic peaks at angles (2θ) 5.6◦, 17.7◦, 20◦, 35◦, and62◦ that were protected, even though they were calcined at300◦C [29, 33]. Other peaks were observed that representedimpurities like feldspar (15%) at the angles (2θ) ∼20.8◦,27.5◦, 41.4◦, and 56.4◦; quartz (2%) at 26.7◦; and calcite (1%)at 29.6◦ [29].

However, at intercalation stage, other than Fe/Cr0.75(D)that is obtained by adding pillaring solution directly tothe dry clay, the samples that were synthesized by bothconventional and ultrasound treatments were found topossess similar basal spacing (d001) values as the 2θ value


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Fe/Cr0.75(CS)-adsFe/Cr0.75(CS)-des.Fe/Cr0.75(SS)-adsFe/Cr0.75(SS)-des.Fe/Cr0.25(D)-ads.Fe/Cr0.25(D)-des.

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P/P0

(b)

Figure 3: (a) Nitrogen adsorption/desorption isotherms. (b) Carbon dioxide adsorption/desorption isotherms of RB and Fe/Cr-pillaredbentonite samples.

of raw bentonite, 15.6–15.8 A (see Table 3), according to X-ray diffraction patterns. These values might result from theformed small Fe/Cr pillars as indicated in the related litera-ture [25]. Furthermore, the identical basal spacing observedfor these intercalated bentonites indicates the presence ofa similar intercalating species in the pillaring solution.However, two peaks can be observed at the reflection angles(2θ) 1.29◦ and 5.64◦ for the Fe/Cr0.75(D), with basal spacingd001 values of 68.5 A and 15.7 A, respectively. These resultswere in accordance with those of the studies related todelaminated iron-intercalated clays [23, 34, 35]. However,the delamination behavior was not observed in the otherintercalated clays, as previously indicated in the literature[36, 37]. Contrarily, Fe/Cr0.25(D) sample showed a decreasein peak intensities and an increase in basal spacing valueswith respect to raw bentonite in the region of 2θ between1 and 10◦. This result indicates that larger Fe/Cr pillars areformed by increasing the chromium content of the Fe/Crpillaring solution [38].

Heat treatment of the raw bentonite at 300◦C for 3 hdecreased the d001 spacing to 14.7 A, indicating retention of

the layered structure at higher temperatures (Figure 2(b),Table 2). Strong electrostatic and other forces between theclay layers can help retain the layer structure of the claymaterials at higher temperatures. On the other hand, aftercalcination at 300◦C, when comparing the patterns of X-raydiffraction of sample synthesized by conventional methodand the patterns of X-ray diffraction of samples obtained byultrasound treatment, distinctive d-spacing peaks were notobserved at XRD patterns of Fe/Cr0.75 and Fe/Cr0.75(CS)samples. The delaminated structure of Fe/Cr0.75(D) waspreserved, while the basal spacing value of Fe/Cr0.25(SS)decreased from 15.8 A to 14.1 A after 300◦C calcination(Table 3).

3.3. N2- and CO2-Adsorption/Desorption Studies. The spe-cific surface areas, the pore volumes, micropore and meso-pore size distribution of the raw, and the pillared bentonitesamples were determined by nitrogen adsorption/desorptionat liquid nitrogen temperature. Carbon dioxide adsorp-tion/desorption isotherms were also performed to determinethe distribution of micropore and mesopore size distribu-tions.


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Figure 4: HK-method pore size distribution curves (a) from nitrogen sorption data and (b) carbon dioxide sorption data and BJH-methodpore size distribution curves (c) from nitrogen sorption data and (d) carbon dioxide sorption data.

Figure 3(a) presents the nitrogen adsorption-desorptionisotherms. Both the pillared and raw bentonites possessedtype II adsorption isotherms, according to IUPAC classifi-cation. The hysteresis found in these materials is H3 type,according to IUPAC classification, and is attributed to either

slit-shaped pores or plate-like particles with space betweenthe parallel plates [30, 31].

The textural properties of pillared bentonites are pre-sented in Table 4. It is seen that all pillared bentonites withFe/Fe + Cr ratio of 0.75 have similar textural properties and


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Figure 5: EPR spectra of the Fe/Cr0.75, Fe/Cr0.75(D), and Fe/Cr0.25(D) samples.

these properties do not change remarkably with differentsynthesis conditions. The similar textural properties of thesepillared bentonites, other than the Fe/Cr0.75(D) sample,may be explained by the formation of a similar sizedbasal spacing and similar dispersions of the pillars in theinterlayer region. However, the Fe/Cr0.25(D) sample hasa greater d001 value than the other samples, but it has alower SBET value, which is associated with the amount anddispersion of the pillars among the interlayers of this sample.Furthermore, this result is supported by the Fe/Cr0.25(D)sample possessing Vm values lower than both the naturaland the other pillared bentonites (Table 4). In addition, whenthe textural properties of the Fe/Cr0.75(D), Fe/Cr0.75(CS),and Fe/Cr0.75(SS) samples are compared, it is seen that thesample Fe/Cr0.75(CS) has the greatest surface area and porevolume values. These results indicated that the micro-poresformation increases by slow addition of the dry clay to thepillaring solution with ultrasound treatment at intercalationstage.

Figure 3(b) shows the carbon dioxide adsorption-de-sorption isotherms of the samples. Both the Fe/Cr0.75(D)and Fe/Cr0.25(D) samples have similar adsorption capac-ities to the raw bentonite, while the Fe/Cr0.75(CS) andFe/Cr0.75(SS) samples have higher values than the othersamples. It was obtained that the adsorption capacities eval-uated from carbon dioxide adsorption data were comparablebut smaller than the corresponding adsorption capacitiesobtained from nitrogen adsorption.

The HK-micropore and BJH-mesopore size distributionsobtained from the nitrogen adsorption isotherms for thesamples that were studied are presented in Figure 4, alongwith the comparison to the corresponding distributions fromthe carbon dioxide data. As can be seen from Figure 4, thereis a good agreement between the pore size distributions ofthe adsorbents studied. Raw and all the pillared bentonitesamples have two modes of distribution of 4.32 A and broad

Wavenumber (cm−1)

40080012001600200024002800320036004000Tr

ansm

itta

nce

(a.

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Raw bentonite

Fe/Cr0.25(D)

Fe/Cr0.75(SS)

Fe/Cr0.75(D)

Fe/Cr0.75

Figure 6: FTIR spectra of raw and pillared bentonite samples.

5–8 A (Figures 4(a) and 4(b)). This behavior indicates thepresence of two types of micropore. As clearly shown inFigure 4, no important increases to the volume adsorbed bythe micropores resulted from the pillaring processes.

The BJH-mesopore size distribution peaks obtained fromnitrogen adsorption are relatively narrow, which allowedus to use the average pore diameter on the maximumof the pore size distribution as the characteristic param-eter (Figure 4(c)). However, a larger BJH-mesopore sizedistribution dispersion is observed for the carbon dioxideadsorption isotherm (Figure 4(d)). It was observed that thepore volumes given by nitrogen isotherms were greater thanthose of carbon dioxide. The observed differences may becaused by differing sensitivities of the isotherms to differenttypes of porosity; for example, nitrogen adsorption at 77 Kmay not be as sensitive as carbon dioxide adsorption at 273 Kto the same types of microporosity when the micropore sizedistribution is large [39].

3.4. EPR Analysis. Two characteristic Fe3+ signals wereobserved in smectite-type minerals with values of g equal to4.3 and 2.0 in the EPR analysis (Figure 5). The presence ofisolated Fe3+ [19, 40] with either tetrahedral or octahedralcoordination is associated to the signal of g equal to 4.3,which for smectite minerals is associated with iron locatedinside of the clay sheets (iron exchanging aluminum in theoctahedral layers). In contrast, the presence of clusters ofiron corresponds with the signal of g equal to 2.0. In thiswork, the signal observed at a g of 2.0 did not change basedon the process and the employed Fe/Fe + Cr ratio, but theintensity of the signal observed at a g of 4.3 was greater forthe Fe/Cr0.25(D) sample.


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32–5

.05

Fe/C

r0.7

5(C

S)10

667

840.

143

0.02

00.

108

0.02

80.

062

0.01

351

.96

33.8

237

.38

13.3

64.

32–5

.30

4.32

–5.0

5Fe

/Cr0

.75(

SS)

104

6582

0.13

90.

019

0.10

40.

026

0.06

00.

012

53.6

833

.82

37.3

813

.36

4.32

–5.3

04.

32–5

.05

Fe/C

r0.2

5(D

)87

6296

0.13

70.

011

0.13

10.

015

0.04

90.

007

62.9

832

.53

37.5

813

.33

4.32

–4.7

24.

97–6

.82


Wavenumber (cm−1)

32003400360038004000

Tran

smit

tan

ce (

a.u

.)

60

80

100

120

(a)

Wavenumber (cm−1)

16002000240028003200

Raw bentonite

Fe/Cr0.25(D)

Fe/Cr0.75(SS)

Fe/Cr0.75(D)

Fe/Cr0.75

(b)

Wavenumber (cm−1)

18001850190019502000

(c)

Figure 7: FTIR spectra of raw and pillared bentonite samples after benzene adsorption: (a) 4000–3200 cm−1, (b) 3200–1600 cm−1, and (c)2000–1800 cm−1.

3.5. FTIR Spectrums. Figure 6 shows the FTIR spectra (400–4000 cm−1) of both natural and all pillared bentonites. Thewave numbers and assignments of the main vibrational peakscome from values found in the literature [41–43]. Therewere few IR spectral differences between the natural andFe/Cr-pillared bentonites which implied that the modifiedclay material kept its original structural configuration afterpillaring. The appearance of a broad peak at approximately3435 cm−1 corresponds to the O–H stretching mode ofthe absorbed water, with a simultaneous shift to lowerwavenumbers and an increase in width. This behaviorindicates an increase in the interlayer water content due tothe replacement of inorganic cations with the Fe/Cr pillars[24, 42]. Also, the decrease of the peaks at 3631 cm−1 canbe detected for all of the pillared bentonites, suggestingthat the Fe/Cr pillars could link with Al–O in the aluminaoctahedral sheet. The peak at 1640 cm−1 can be associated tothe bending vibration of the water and shifted to 1636 cm−1

after pillaring. Also, a shift of the peak of raw bentoniteat 1036 cm−1 towards higher frequencies was nearly the

same in all pillared bentonites (1044 cm−1). This change hasbeen previously related to a change in the symmetry of thesurface Si–O–Si vibration, which is perhaps associated witha change in the electric field near the Si groups due to theproximity of the more positively charged Fe/Cr pillars [44].Also, the bands at 526 and 467 cm−1 can be ascribed tothe Al–O stretching and Si–O bending vibrations shifted tohigher wavenumbers (529–469 cm−1) all pillared bentonites.Absence of additional peaks suggests that no bond formationoccurs between bentonite and Fe/Cr-pillars [45].

Adsorption of benzene shows that no strong acid sitesare present in all samples (Figure 7). However, because ofthe interaction between benzene and silanols, there is a smallpeak observed at 3756 cm−1 for all samples (Figure 7(a)). Itwas observed that the peak intensity at 3756 cm−1 increasedslowly after pillaring with the Fe/Cr pillars. Furthermore, itis seen that after benzene adsorption the peak at 1640 cm−1

increased in intensity (Figure 7(b)). There are two smallpeaks observed in the all samples, at 1831 cm−1 and1968 cm−1 from the C–H out-of-plane bending vibration


Temperature (◦C)

0 100 200 300 400 500 600 700 800

Mas

s lo

ss (

%)

−20

−15

−10

−5

0

Fe/Cr0.75(D)Fe/Cr0.75(SS)

Fe/Cr0.25(D)

Figure 8: TG curves of some pillared bentonite samples.

(Figure 7(c)) [43]. The increase in peak intensity is moredistinctive at 1640 cm−1, especially for the Fe/Cr0.75 andFe/Cr0.75(SS) samples. This result indicates that the aciditiesof these samples are greater than the others. After benzeneadsorption to the pillared bentonites, the changes in the FTIRspectra implied that there are a few acid sites in the pillaredbentonites.

3.6. Thermogravimetric Analysis. In order to investigatethe thermal behavior of the air-dried Fe/Cr0.75(D), Fe/Cr0.75(SS), and Fe/Cr0.25(D) samples, the thermogravi-metric (TG) analyses were carried out in the temperaturerange of 25–800◦C. Pillared bentonites show similar weightloss behavior in the TG analysis (Figure 8). The pillaredbentonites show high initial weight losses which graduallydecrease at higher temperatures. Two weight loss regionshave been observed for all samples in the range of 25–300◦C and 300–700◦C. The major weight loss occurred at25–300◦C is due to the removal of water molecules presentin the interlayer. This dehydration process is one of thecharacteristics of clay minerals [46]. However, the weightlosses observed in the range of 300–700◦C are attributedto the water molecules coordinated to the pillars as well asthe dehydroxylation of the pillars and clay sheets and canbe correlated indirectly to the quantity and distribution ofthe Fe/Cr-pillars. Contrarily, the total weight loss with theFe/Cr0.25(D) sample is higher than that of the Fe/Cr0.75(D)and Fe/Cr0.75(SS) sample, which suggests that a significantlyhigher amount of water is present in the micropores of the

two samples. The Fe/Cr0.75(D) and Fe/Cr0.75(SS) samplesshowed similar total weight losses.

3.7. Catalytic Activity for the Wet Peroxide Oxidation of Phe-nol. After the synthesis and characterization, the obtainedFe/Cr-pillared bentonites were tested for catalytic activityfor the catalytic wet hydrogen peroxide oxidation of phenolunder the conditions defined previously. For the purposeof comparison, a control reaction was conducted under thesame conditions using only raw bentonite. The quantityof phenol removed using RB, Fe/Cr0.75, Fe/Cr0.75(D),Fe/Cr0.75(SS), and Fe/Cr0.25(D) in the presence of H2O2 ispresented in Figure 9. Comparing the oxidation data of theraw and pillared bentonites reveals that pillaring effectivelyincreases the activity of the bentonites. This catalytic activitycan be mainly related to the metal content and textural prop-erties. The catalytic activity increases for the solids treatedwith the active metals. When the oxidation activities of sam-ples synthesized via the conventional method are comparedto the activities of samples synthesized by ultrasonication, thesample synthesized by the conventional method, Fe/Cr0.75,showed a higher activity than the other pillared samples(Figure 9). The Fe/Cr0.75 sample resulted in a conversionof 50% of the phenol after 3 h. The phenol conversions ofthe Fe/Cr0.75(D), Fe/Cr0.75(SS), and Fe/Cr0.25(D) sampleswere 34, 26, and 40%, respectively over 3 h. The total organiccarbon conversion was 3, 31, 26, 26, and 30% for RB,Fe/Cr0.75, Fe/Cr0.75(D), Fe/Cr0.75(SS), and Fe/Cr0.25(D)catalysts, respectively, over 3 hours. According to theseresults, proper number of active sites and their distributionon the catalyst surface is more important for catalystactivity in TOC conversion than the metal content in Fe/Cr-pillared bentonites. Sample Fe/Cr0.75(SS) with fairly highiron and chromium content has the small activity as theother pillared bentonites with lower metal content. Also,these results indicate that, for the complete degradation ofphenol under the specified condition, a 3-hour reactiontime is inadequate for Fe/Cr0.75 and Fe/Cr0.25(D) samples.The catalytic behavior in phenol oxidation in pillared clayshas been recently reported in the literature. In this regard,Carriazo et al. [47] found 100% phenol conversion in lessthan an hour, and high TOC conversion value (55%) usingAl-Ce-Fe-pillared clays. Similarly, Sanabria et al. [48] foundtotal fenol conversion and high TOC conversion (49–53%)in two hours using Al/Fe-pillared clays.

To verify the chemical stability of the materials thatwere synthesized during this investigation, the presence ofinorganic cations in the reaction mixture was analyzed.The obtained AAS confirmed that the iron concentrationsin the filtrates taken after 3 h of the reaction did notexceed 0.11 ppm. This value was lower than those reportedfor Al, Al-Fe-, Al-Ce-Fe-, and Al-Fe-Ce-pillared clays syn-thesized by the conventional, ultrasound, and microwavemethod [6, 19, 49], thus demonstrating the stability ofthe active phase. However, chromium concentrations forthe Fe/Cr0.75, Fe/Cr0.75(D), and Fe/Cr0.25(D) sampleswere observed as 24.08 ± 0.34, 24.53 ± 0.16, and 80.13± 1.27 ppm, respectively, indicating that iron pillars aremore stable than chromium pillars. However, the catalytic


t (min)

0 60 120 180 240

Ph

enol

con

vers

ion

(%

)

0

10

20

30

40

50

60



(a)

0

10

20

30

40

50

60

t (min)

0 60 120 180 240



TO

C c

onve

rsio

n (

%)

(b)

Figure 9: Catalytic activity for the raw and pillared bentonites with ultrasound and conventional method. (a) Phenol conversion. (b) TOCconversion at atmospheric pressure and at 25◦C.

behavior in catalytic wet peroxide oxidation of phenol inFe/Cr-pillared clays has not been recently reported in theliterature.

4. Conclusions

The synthesis of Fe/Cr-pillared bentonites starting fromeither a 2% clay suspension or dry clay and using ultrasoundtreatment during both the aging and intercalation stepsof the pillaring solution allows a considerable decrease inthe volume of water and the synthesis times comparedwith the conventional synthesis method. The results of theXRD and adsorption-desorption isotherms analysis showedthat pillaring caused a small increase in basal spacing(d001) (except of occurring delamination synthesized byultrasonic treatment and sample that is directly added tothe pillaring solution of dry clay), surface area, and porevolume values. TG analysis shows two stages of weight lossdue to dehydration-dehydroxylation processes. In addition,the results of the characterizations and catalytic tests carriedout for the Fe/Cr-pillared bentonites synthesized by meansof ultrasound treatment displayed similar physicochemicalcharacteristics and smaller phenol conversion than theFe/Cr-pillared bentonite synthesized by the conventionalmethod in this study.

Acknowledgment

This research was partially supported by the Scientific Re-search Project Department of Mehmet Akif Ersoy University(Project code 0107-NAP-10).

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